Digital optical communication systems, such as local area networks, may utilize either single mode or multimode fiber optic links as transmission media. Single mode fiber, primarily used for long-haul transmission, has a very small core and usually requires coherent light for input signals because of the very small aperture of the fiber and the need to capture enough light. This is particularly important where there is a high data transmission rate over a long distance, and the added chromatic dispersion of the broadened spectrum from the light source is detrimental to the system. Coherent light is the normal line spectrum of a laser; a light-emitting diode (LED) could not provide sufficient optical power for high speed, long distance transmission through single mode fiber.
Multimode fiber, commonly used for short-haul transmission, has a much larger core than single mode through which multiple modes of light can pass. In order to avoid the problem of modal noise, multimode fiber requires incoherent light which is usually produced by LEDs. Modal noise results from the interference of pulses of coherent light with one another over the length of the fiber cable. The coherency of laser light can be destroyed by changing the index of refraction of the laser cavity, which in turn shifts the wavelength slightly such that the normal line spectrum of the laser is blurred. This invention provides a means of using a laser and a minimal circuit to produce the coherent light needed for single mode fiber or the incoherent light required for multimode fiber.
The known prior art that utilizes a laser as a source of incoherent light for multimode fiber employs additional components not included in the present invention. In U.S. Pat. No. 4,317,236, a separate microwave oscillator is used to destroy the coherency of laser light, in order to reduce the modal noise in multimode fiber. The connection between the oscillator and the laser current driver can lead to electromagnetic interference problems. Furthermore, the oscillator has its own timing device which may not correspond to the timing of the laser and will slow down the maximum data transmission rate. Use of the oscillator adds cost to such a laser driver.
Both a high frequency oscillator and an amplitude modulator are employed to destroy coherent light in U.S. Pat. No. 4,611,352, a device that performs long-haul transmission of a signal using a laser and multimode fiber. As compared to the present invention, the use of those components will increase cost and electromagnetic interference, while decreasing data transmission speed.
A third device in the relevant prior art, U.S. Pat. No. 4,603,421, is a multi-emitter laser that provides an incoherent composite laser input beam to multimode fiber in order to eliminate modal noise. Unlike the present invention, the emitter requires a special array laser coupled to the fiber to approximate an LED, and does not operate on single mode fiber.